The Price List field is an interesting field in that it is a field has script that fires from it but you cannot see it from the form editor where you place your own scripts. I recently tried removing this field from an Opportunity and the Account form. The Account didn't show any repercussions but the Opportunity wouldn't allow me to add an Opportunity Product to the Opportunity.
With the Price List field missing I just got this random non-descript scripting error. When I added the field back and tried to add an Opportunity Product I received the error I expected of having no Price List set. The problem now became that when I tried to select a Price List for the Opportunity I couldn't see any Price Lists to choose from. Basically the script that was running and populating the filtered list of Price Lists had been removed when I removed the field from the form and it doesn't come back when I put it back onto the form.
The question now became how do I fix it? How do I get that script back. I have to give credit to Nahi for his post in the Microsoft.Crm.Public group who ran into this issue and posted the solution he found. He didn't go into the details, he just went over the process. I'm going to try and give you a little bit more.
Basically the idea was, grab the script from another entity or another system where the Price List had not been removed. The way to do that is to export the Opportunity and look through the entity XML for the missing script. Then copy and paste the script XML into the entity XML of the entity that had the Price List field removed from it. Save changes and reimport it back into CRM, publish and you're set. It worked great for me and I now no longer have any issues with the Opportunity, Price List or adding Opportunity Products.
The best way to find the script from the clean entity XML is to search for "Price List" and then look through to where you see the script next to one of your finds for "Price List". Here's the XML block that I found and added (it should go between the LABEL and CONTROL tags):
<event name="setadditionalparams" application="true" active="true">
var oLookup = event.srcElement;
<dependency id="transactioncurrencyid" />
Hope this helps someone else out.
Dynamic Methods Inc.